Hanuman Jayanti is celebrated on full moon day during Chaitra month (current year celebration April 4th, 2015). Hanuman, who is also known as Vanara God, was born on this day and Hanuman Jayanti is celebrated to commemorate the birth of Hanuman.
Lord Hanuman, a dedicated disciple of the Hindu Lord Rama, is worshipped all over the world by Hindu people for his deep devotion to the god Rama. At the day of Hanuman Jayanti celebration, all the Hanuman temples become full of crowd as people started puja from early morning after a holy bath. Hanuman Jayanti is celebrated with great enthusiasm by the people of Hindu religion as an important festivel of the Hindus.
Additional information on this auspicious event can be found > Here